It will be understood that there are two aspects to the acidity problem: first, the problem of stopping or significantly retarding the acid producing reactions and second the problem of maintaining the leachate at an environmentally acceptable pH.
At many mines, precipitation water passes through a mass or seam of an acidity-causing mineral such as pyrite (i.e. ferrous sulphide,FeS2) and other sulphide minerals (e.g. CuS, ZnS, FeS). The leachate from such seams may have an environmentally unacceptable pH. The leachate may pass directly into a natural drainage system or indirectly through a body of acid surface water which has collected in a quarry produced during mining.
Pyrite readily oxidizes in the presence of air and water in a complex manner according to a series of reactions which, in total, may be summarized in terms of the following simplified stoichiometric relationship: ##STR1##
The resulting leachate is acidic, as indicated by the presence of sulphuric acid in the equation.
The iron in the pyrite changes from a reduced state (i.e. Fe++) to an oxidized state (i.e. Fe+++) in the hydroxide, and this change is in itself acid-producing; and by a feedback mechanism this change causes further oxidation and therefore further acidity.
These types of reactions are not confined only to iron. Where the mineral includes a metal other than iron, similar reactions which change the metal to a more oxidised state similarly produce further acidity. The pH in the leachate waters which can be attributed to these mechanisms can be 3 or even lower in a typical real case.
Acidity is dangerous not only in the collection pond in old workings but also because it may be a hazard to a local natural drainage system.
Such acidity can occur wherever precipitation water seeps through exposed pyrite and equivalent minerals. The acidity does tend to disperse eventually, as natural bases dissolve in the acid and by dilution as metals and other substances dissolve in the acid. However, this could take centuries, and relying on natural dispersion is unacceptable in most mine situations. Besides, in many cases, the new water entering the pond, having passed through the exposed pyrite, is also acid, and is not an effective diluent for the acidity of existing waters.
It has been proposed to treat acid pond water with continual additions of bases such as lime, Ca(OH)2. Such treatment systems in general require high maintenance costs, which must be continued, it is estimated in some cases, for hundreds of years.
It should be borne in mind also that, by using a treatment system which attempts to treat the water in the pond, then no matter how effective that treatment system might be the water entering the pond will still be acidic, and the treatment will have to be continued. The cause of the acidity in the pond is the fact that the water entering the pond has passed through expose pyrite, or other acid-producing mineral.
As a way of preventing the acidity-producing reactions from taking place, it has been proposed to seal the acid-generating minerals (pyrite or other) from exposure to atmospheric oxygen. This can be done, for example, (though at great expense) by building a dam and flooding the exposed seam of pyrite; at even greater expense, the exposed portions of the acid-producing mineral can be excavated from the ground, and dumped in a deep lake, with unknown environmental consequences.
Pyrite can become exposed naturally as a result of geological action, but pyrite, once exposed, naturally oxidizes and its acidity-causing properties disappear fairly quickly (as measured on a geological time-scale). In fact, acidity problems arising from naturally exposed pyrite are rarely found: in ponds or lakes that have acidity problems, the problems usually are of man-made origin.
There are of course many chemical reactions known which theoretically--that is to say, in the laboratory, or on a small scale--can serve to cause dilution of acids in water.
However, these reactions have not proved suitable to be economically practised in a body of water having an area which may be measured in the hundreds of hectares. Furthermore, any system of treatment which involves taking the water out of such a pond to be treated will almost certainly be unacceptable on cost grounds.
The principle of the galvanic cell is known, in which two conductors are placed in an acidic electrolyte, the two conductors having different standings in the electro-chemical series; if a circuit is established between the two, under the right conditions a current will start to flow. Free electrons become available at the conductors (electrodes), and convert hydrogen ions in the electrolyte to hydrogen gas, which bubbles away. As a result, the pH is raised, and the acidity is diluted. Adding electrical energy into the circuit from outside can be arranged to serve to increase the acid-diluting activity.
It is an aim of the invention to utilize the principle corresponding to that of the galvanic cell actually in the ground, for the purpose of preventing the natural acidity-producing reactions from taking place.
U.K. Patent Application GB2 202 862A (Ontario, January, 1987), U.S. Pat. No. 4,678,554 (Eltac, July, 1987), U.S. Pat. No. 3,823,081 (Kettering, July, 1974), U.S. Pat. No. 4,561,948 (Stiller, December, 1985), Russian Patent 1556589 (Russkikh, no date), are examples of prior publications, in which what might be called a galvanic cell principle is utilized. Three of these--Ontario, Eltac and Russkikh have nothing to do with the treatment of acid mine drainage, nor with the retardation nor prevention of acid generating reactions.
U.S. Pat. No. 4,561,948 (Stiller, December, 1985) is an example of a prior publication of a small scale system for treating acidity, in which what might be called a galvanic cell principle is utilized to treat acidity. While such a system might be acceptable for, say, protecting the water drawn off from a well, it is wildly uneconomic for use on a large body of water. The cost of providing a large enough vessel in which to carry out the treatment, alone is prohibitive.
U.S. Pat. No. 3,823,081 (Kettering, July, 1974) is a second example of a prior publication, in which what might be called a galvanic cell principle is used to treat acidity. It relies upon pre-treatment of mine water before the water is pumped into an electolytic cell contained within an artificial medium such as glass, plastic or cement. It does not use the acid generating mineral as an electrode, both anode and cathode are artificial materials and an external power source is used. It is uneconomic for a large body of water and does not prevent acid generation.
It may be noted that the Stiller patent is concerned solely with the treatment of water that already contains a high concentration of acid. The treatment system of the invention may also be used for treating an already contaminated pond of water (although, unlike Stiller or Kittering, in the invention the water is treated while remaining actually in the pond). The invention, however, is aimed more at preventing the acidity-producing reactions from taking place at all, an aspect which is not addressed in Stiller or Kettering.
In an optional alternative application a body of acid-generating mine tailings is converted into the cathode of an electrochemical cell by the insertion of a grid or grids of metal mesh. Ontario and Eltac also employ meshes, but they are employed as electrodes themselves. In the case of the invention, the mesh is merely a way of incorporating the tailings into an electrochemical cell. In other words the mesh in the invention acts simply as an electrical lead or connector, not as an electrode. The tailings constitute the true electrode.
It is an aim of the invention that the treatment of the water should take place with the water remaining actually in the ground.
It is an aim of the invention that the seepage water emerging through the exposed pyrite or other mineral, in the ground, should contain only an acceptably low concentration of acid.
It is also an aim of the invention to provide a treatment system which can be left to operate by itself, with little or no maintenance. It happens sometimes that mine workings are not just closed, but are totally abandoned, with no resources left for such things as cleaning up acidity. It is an aim of the invention to provide a treatment system which can be put in place economically in a mine in which such total abandonment is contemplated, being a treatment system which can be expected to keep acidity effectively under control indefinitely, with minimum or zero maintenance.